mpi3mr: Assume dma_hiaddr is BUS_SPACE_MAXADDR

No sense having a variable for this. So use BUS_SPACE_MAXADDR and remove
dma_hiaddr from softc.

Suggested by: jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42808

(cherry picked from commit 1ec7c672bc2854a1efb1d50f189d4881163c16c6)